Résumé: In a process called 'base rivalry', irreparable mutations are provoked in the replication of monotonous sequences, which depend on the cell temperature, the cell viscosity and monotonous sequence length. This explains the very long monotonous sequences and very long DNAs that occur over long evolutionary epochs. Presumably, base rivalry (with tautomerism or too low cell viscosity) also provokes the formation of tumors and the emergence of dangerous viral mutations.